Cause of Historic Ocean Heat Wave off San Diego in 2018

Climate researchers at Scripps Institution of Oceanography at UC San Diego identified a new main reason why water temperatures off San Diego reached their highest point in at least a century in 2018.

Previously physical oceanographers at Scripps had pointed to an unusual absence of winds and clouds off the coast as the reason for surface waters warming under the hot summer sun and reaching a temperature of 26.4℃ (79.5℉) at the heat wave’s peak. Three times during August 2018, a new record high water temperature was established, shattering a record that had stood for nearly 90 years, according to data that Scripps Oceanography has collected since 1916.

Research led by Xinyue Wei and Kaiyuan Li, who served as interns with Shang-Ping Xie, a climate scientist at Scripps Oceanography, during a 2019 international undergraduate research program, put a new spin on the event. They found that weather events thousands of miles away from California were the dominant force that essentially stilled local waters enough to let them warm up over a period of several weeks.

Noticing that the warming lasted almost the entire summer, from July through September, the team divided the periods of ocean temperature deviations into sub-monthly (fewer than 30 days), subseasonal (30-120 days), and longer time scales. They found that the subseasonal warming was due to ocean fluctuations that came from the equatorial Pacific, hugging the coast and deepening the upper warm layer. In the week leading to the record heat wave, remote phenomena including tropical storms several hundred miles to the south of San Diego stifled the flow of deep, colder water to the surface, and thickened the thermocline, the layer of the ocean that is nearest to the surface and warmed by sunlight.

The conclusion from Xie’s group highlights the large-scale environment in which the coastal marine heat wave occurred. Scripps colleagues had earlier identified a lack of cloud cover and curtailed winds as the source of the warm ocean off San Diego in a 2020 study. The two results reflect the different approaches to observing the natural world between seagoing oceanographers more concerned with direct ocean measurements and climate scientists who look for global connections  of climate events.

“Coastal waters off California are usually cold even in summer,” said Xie, who directed the research. “This study shows that they can be as warm as Hawaii when waves from the equator and the relaxed winds conspire to shut off the upwelling that pumps cold water from beneath.”

James Fumo, now a graduate student at the University of Hawaii, led the 2020 study and said its goal was to describe the phenomenon and document that a long-term trend of warming caused by human activities such as fossil fuel burning raises the baseline for extreme heat events. Fumo and colleagues concluded that with so much heat energy added to the oceans in general, any warming episode has a substantially improved chance of setting records. 

“The 2018 marine heat wave brought unprecedented warming to the Southern California/ Northern Baja region,” Fumo said. “We are glad that our efforts to describe this event and it’s links to climate change have been built upon by Xie. This new study greatly improves our understanding of the drivers of this marine heat wave.”

Xie said he agrees with that premise and that his work illustrates how events in one region influence events in other regions even over great distances.